1. Field of the Invention
The present invention relates to positioning systems. More specifically, the present invention relates to systems for the detection and correction of spatial errors in the position or alignment of an industrial apparatus.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
In manufacturing, a variety of apparatus need to be positioned or aligned with respect to a reference or ideal position. Such alignment may be necessary to process a plurality of work pieces and ensure consistency and quality of manufacture. Accordingly, industrial positioning systems have been developed to provide automated positioning of a work piece relative to a tool or other implement. Such industrial positioning systems typically include error detection and correction systems to optimize the performance of the positioning system.
A common type of error detection and correction system adjusts the position of the work piece in response to an error signal. The error signal is often an analog signal which is developed by a detection device as a linear function of the spatial error between the actual position of the piece and a desired or reference position. As the spatial error decreases, the amplitude of the error signal decreases. Ideally, power to the positioning system should decrease to zero as the amplitude of the error signal decreases to zero. Should the piece subsequently become misaligned, an error signal having the same linear proportionality to the spatial error would again be developed. Through feedback, the stability at the reference position is ideally maintained.
Unfortunately, in most mechanical systems, static and dynamic friction operate to impair the performance of the system. For example, in the typical industrial positioning system, the amplitude of the error signal, for relatively small spatial errors, may be insufficient to supply enough power to the mechanical driver to overcome static friction in the system. Thus, notwithstanding the detection of a spatial error in the alignment of the piece and the development of an appropriate error signal, the misalignment may not be corrected as the signal level may have been too low to overcome the effect of friction.
Mechanical and electrical solutions have been proposed to counteract the effects of static friction. These approaches have tended to either impair the performance of the system or substantially increase the cost thereof. For example, an electrical solution to the problem compensates for static friction by adding a constant offset voltage to the error signal. An inherent limitation of this approach is due to the fact the offset voltage is constant and monopolar regardless of the error signal amplitude and sign.
A further limitation on this approach is due to the fact that the offset voltage has to be precisely calibrated to the mechanical backlash of the system in addition to the static friction. (Mechanical backlash results from an overshoot of the desired position caused by momentum in the mechanical implement or piece under movement. This causes another error signal to be generated which forces the implement or piece to be moved rapidly in the reverse position toward the desired position.) Calibration is critical as the presence of the offset voltage could otherwise cause the summed voltage to have a zero amplitude even though there is a spatial error. Yet such precise matching requires costly and extensive field adjustment by trial and error.
These problems are particularly acute in certain applications, such as focused ion beam semiconductor manufacturing systems, requiring a high degree of accuracy in the alignment of a work piece or sample relative to an implement or tool. There is therefore a need in the art for an inexpensive yet effective system and/or technique for controlling the position of a work piece or sample relative to an industrial implement.